Polypropylene possesses several highly desirable properties such as low specific gravity, high heat resistance, good chemical resistance, excellent impact resistance and high stiffness. Because of these desirable properties, polypropylene homopolymers and copolymers find extensive use in industry, with homopolymers being used in approximately 80% of all applications. Homopolymers are generally produced having melt flow rates (MFR) ranging from less than 1 to greater than 100, with lower MFR grades used for sheet, film and general purpose extrusion and the higher MFR grades finding utility in injection molding and fine denier fabrics. Random copolymers are produced by introducing small amounts of ethylene, normally about one to seven percent by weight, into the polymerization reaction. The product of the well-known random copolymerization process exhibits improved clarity and toughness, with stiffness reduced somewhat over the pure homopolymer.
About one-half of all polypropylene produced is processed via extrusion techniques, with the manufacture of fiber and filament as slit film, monofilament and multifilament being among the higher volume applications. Polypropylene homopolymers and copolymers can also be extruded into sheet or film form, with MFR's below three commonly employed. However, these resin sheets and films have hardly been used on a commercial scale for thermoforming due to problems encountered when attempting to thermoform such films and sheets. As noted at page 72 of the Modern Plastics Encyclopedia, J. Arganoff, Editor, McGraw-Hill Inc., 1985: "Sheet applications have traditionally been limited by poor melt strength, which makes [polypropylene] difficult to thermoform using conventional techniques..revreaction. The poor melt strength characteristic of polypropylene causes the sheets and films made therefrom to sag when heated during the thermoforming operation.
To address the problems associated with thermoforming polypropylene, costly high melt strength polypropylene resins have been developed specifically for thermoforming applications. Also, the recent development of modified thermoforming processes, such as the Shell-developed solid-phase pressure-forming process (SPPF) which operates below the melting point of polypropylene, or the differential pressure forming method of U.S. Pat. No. 4,666,544, has increased the use of polypropylene in thermoforming.
Japanese Patent Publication No. 14575/74 provides another proposal for the use of polypropylene in thermo-forming. This reference discloses a process for the heat molding of crystalline polyolefin resins in solid form at a temperature lower than their melting point for the purpose of improving the transparency of the products so produced. This process is said to be particularly effective in heat molding polypropylene; however, a lowered effectiveness is exhibited when the products are prepared by molding comparatively high molecular weight resins having an MFR of four or less. Thus, to eliminate such a disadvantage, it is necessary to use resins which have a high MFR and are therefore very disadvantageous in the area of impact resistance. The use of a thermoforming temperature lower than the melting point of polypropylene, while certainly better than the use of a thermoforming temperature higher than the melting point, will not, in and of itself, thoroughly eliminate such sagging.
Still another attempt to eliminate the sagging of resin sheets at the time of thermoforming is disclosed in Japanese Patent Publication No. 75761/76. In this invention, a polypropylene sheet is laminated onto a sagging-free sheet of a resin different from polypropylene in attempts to solve the problem of sagging; however, this may be unsuitable for general use since it raises problems as to lamination means, selection of resins used and the like.
Despite these developments, it would be highly desirable, especially from an economic standpoint to utilize conventional polypropylene resins to form films and sheets which are thermoformable using conventional thermoforming processes. Therefore, what is needed is a process for producing a polypropylene film or sheet which may be thermoformed using conventional techniques and equipment.